Receivers are widely used in modern technology devices, as portable devices, computers, phones, etc. Wireless communication devices form a large majority of applications using portable radio devices. Usually, the receivers (RX) are coupled with transmitters (TX) in modern communication devices.
In wireless communication one of the major challenges is that the RF receiver i.e. the receiver which handles the input radio frequency signals, should tolerate the strong out-of-band interference when they receive a weak wanted signal. For example, in WCDMA applications, TX and RX operate simultaneously. Due to limited TX-to-RX isolation in the TX band, it is a relatively large TX leakage in the TX band which in turn determine stringent RX out-of-band Input Intercept Point 3 (IIP3) and IIP2 requirements. These requirements are on top of the low noise figure requirement. In order to mitigate the out-of-band linearity requirement of the RX front-end, an inter-stage Surface Acoustic Wave (SAW) filter between the LNA and the mixer is usually added to further suppress the TX leakage, as it is shown in FIG. 1.
Similar situations are found in a GPS receiver where the GPS has to co-exist with WCDMA and other applications.
A lot of effort has already been made to eliminate this SAW filter, for lowering the cost and for reducing the size of the receiver. N. Yanduru et al., “AWCDMA, GSM/GPRS/EDGE receiver front end without interstage SAW filter,” in RFIC Symp. Dig., June 2006, pp. 77-80, employed a high Q LC tank circuit made with the help of the bonding wire at the LNA load to further attenuate the TX leakage, and thus improve the out-of-band IIP3 and IIP2. B. Tenbroek et al., “Single-Chip tri-band WCDMA/HSDPA transceiver without external SAW filters and with integrated TX power control,” in IEEE ISSCC Dig., February 2008, pp. 202-203, adopted a tuned Q-enhanced LC notch filter at the intermediate node of the LNA for rejecting the TX leakage. Both approaches need many inductors for different bands, and might be impractical for those WCDMA bands with small frequency duplex distance. Without extra inductor, H. Darabi, “A blocker filtering technique for SAW-less wireless receivers,” IEEE J. of Solid-State Circuits, vol. 42, no. 12, pp. 2766-2773, December 2007, demonstrated a blocker filtering technique using a translational loop. However, it sacrifices the performance of power consumption and noise figure.
Recently, D. Kaczman et al., “A Single-Chip 10-Band WCDMA/HSDPA 4-Band GSM/EDGE SAW-less CMOS Receiver With DigRF 3G Interface and +90 dBm IIP2”, IEEE J. of Solid-State Circuits, vol. 44, pp 718-739, March 2009, proposed an elegant solution using a single stage RF trans-conductance amplifier (TCA) followed by a passive current mixer whose switches are controlled by a Local Oscillator (LO) whose wave forms provides a 25% duty-cycle switching of the mixer. The circuit concept is presented in FIG. 2. With this schematic, a large out-of-band IIP3 is achieved because it is no high voltage swing at the output of TCA, and hence less linearity degradation compared with the conventional Low Noise Amplifier (LNA). Adding a low-pass filtering before converting to voltage by the trans-impedance amplifier (TIA), the total RX out-of-band IIP3 is significantly improved. Furthermore, switching with 25% duty-cycle LO leads to 3 dB higher conversion gain, better noise figure, and lower flicker noise than that switching with 50% duty-cycle LO in the mixer. Since the output of the RF TCA is a current, it is not possible to achieve input matching using shunt-shunt feedback as in the conventional voltage-mode LNA. Generally an inductive source degeneration circuit is adopted for improving the low noise figure, as shown FIG. 3 (a). For multi-standard applications, multiple inductors are needed, which determines a dramatic increase of the chip area. Common gate input is also used for input matching, as it is shown in FIG. 3 (b) and (c). However, these implementations provide a high noise figure.
Additional disadvantage for those input matching circuitry is that the trans-conductance of the TCA (or LNA) has to be fixed over different gain, resulting in high current consumption over all the gain settings.